Vinylgold species/intermediate

The first gold-catalyzed addition reactions of carbon nucleophiles to allenes were only first disclosed in 2006, and the number of examples is still small. Toste and co-workers showed that allenic silyl enol ethers undergo a 5-endo- ng cyclization to hexahydroindenone derivatives in the presence of a cationic gold catalyst (Scheme 4-10). In these transformations, water or methanol is used as an external proton source for protodeauration of an intermediate vinylgold species. In an analogous manner, cyclopentenes were obtained in good yields from allenic P-ketoesters. In the presence of a palladium catalyst and an allylic halide, these substrates afford functionalized 2,3-dihydroflirans. 

The use of internal propargylamides also allowed the first isolation of stable vinylgold intermediates derived from alkynes [67]. The simple trick of capturing the proton, which would decompose the vinylgold species by protodeauration, by a simple base as triethylamine, worked beautifully (Scheme 7). This principle could be extended to other reaction types as hydroalkoxylation and hydroaminations [68]. 

With the electTOTi-poor allenic esters, palladium(0) is able to catalyze the reaction without gold. The reactiOTi then is initiated at the other end, after oxidative addition of the aryl halide to the electrophilic palladium(II) species cycloisomerizes the allenic ester and then forms the product by reductive elimination. With o-alkynylbenzoates, the intermediate vinylgold species contains an enol ether substructure and is able to directly intercept the activated allyl donors, even in the absence of palladium. In both cases, by careful trace analysis (ICP), the presence of the other metal was excluded [78]. 

Whereas these transformations require stoichiometric gold compounds, catalytic amounts of both gold and palladium are sufficient for the cycloisomerization of allyl allenoates to allyl-substituted butenolides. Blum and co-workers reported this tandem C-O/C-C bond formation, which is initiated by activation of the distal allenic double bond with PhaPAuOTf (Scheme 4-107). This induces cyclization to an allyl oxonium intermediate, which undergoes deallylation in the presence of Pd2dba3. Nucleophilic attack of the resulting a-vinylgold intermediate at the ti-allylpalladium species and reductive elimination furnish the allylated butenolide and regenerate both catalysts. 

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